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Static versus dynamic routing

  • Filed Under:
  • Cisco
  • 13 February 2012

Static versus dynamic routing

Static versus dynamic routing.
A router requires a ROUTE to a remote network if it is to send traffic there.
A route specifies how the network can be reached.
Routes are stored in a route table.
The route table can consist of static, dynamic or a combination of routes.
We can examine the route table by issuing the show ip route command.
In static routing the route table is constructed manually by the router programmer.
In dynamic routing the route table is learnt by the router itself.
There are advantages and disadvantages to each method.

Static routing.
Static routing means it doesn’t change.
-in other words if a route is to exist we have to program it into the router
-if a route goes down the router cannot find a way of routing around the problem
-this means a lot of work for the programmer in a larger inter-network since they will haveto program routes to ALL networks in manually
Static routing though is very efficient
-does not increase overheads on WAN lines as there is no need to deploy route table updates to other routes
Static routes could be considered more secure than dynamic ones
-since only networks given routes in the route table can be reached, access to network segments can be restricted.
Static routing is recommended when:
-a network is reachable in one router hop
-or when there is only one route to a network
This is so that the overhead of broadcasts produced by dynamic routing protocols is reduced
To configure a static route we use :-
Ip route


Router A needs to send jobs to router B’s S0 interface in order to reach network 192.0.3.0.
Our command therefor is:-
Ip route
Ip route 192.0.3.0 255.255.255.0 192.0.2.2


We have now added a new network section.
We could add another static route on router A so we have a route to each remote network, or we could add a gateway of last resort (default route)
To add a default route we modify the ip route command to
Ip route 0.0.0.0 0.0.0.0

Default route
A default route simply specifies what a router should do with packets for networks it does not have routes to
In our example I could configure the default route to point to 192.0.2.2 this means I wouldn’t have to configure routes to all remote networks on router A
Router A sends any packets to router B which is then responsible for forwarding them to the final destination

Static routing
If static routing is being used it is advisable to add the ip classless command to the configuration
This allows the router to route packets onto its directly connected networks without the need to configure routes to them all
This saves a significant amount of effort

Testing static routes
There are a number of ways that we can test statically assigned routes:-
-Show ip route – displays routes in the route table and the connected networks
-Ping – sends a packet to the remote host and waits for a response
-Trace - lists the routers a packet travels through in order to reach its destination

Dynamic routing
Dynamic routing means that the router can change without the need for human intervention
Routing protocols are used to learn about routes through the inter-network
This means that new network segments can be added without the need for lengthy reconfiguration
If a route goes down for some reason an alternative route can automatically be utilised
The problem is the overheads required by dynamic routing:-
-WAN links have to carry route table update broadcasts as well as data
-The router requires more processing power and memory
-There are different types of algorithm used by dynamic routing protocols. We are going to concentrate on the distance vector algorithm
-An algorithm defines how the router gets information about the network. Some are more efficient than others

Distance-Vector routing
The distance-vector algorithm is a simple technology that can be deployed in dynamic routing
Routing protocols such as routing information protocol (RIP) use this algorithm
It essentially describes the manner by which routers learn paths through the inter-network
In a system based on the distance-vector algorithm the following occurs:-
-At power on the router will determine any other routers connected to its interfaces
-This information is then stored in the route table
-Every 30-90 seconds (depends on routing protocol) the route table is broadcast to neighbouring routers
-The update received on the remote router will be examined and any new information will be added to its own route tables
-If a new route has been discovered to a network already known to the router it will calculate the best route to that network
-to do this it utilises something known as the metric
-in the case of RIP this is the number of hops to the destination LAN


Since distance-vector determines the best route as the one with the minimum number of hops the route across the 56k line will be chosen
This is not the best choice as it is slow. It would be faster to send data to router C and then on to router B via the 1MB links
In this instance we would need to manually edit the metric

There are other issues involved with distance-vector based protocols. We will examine these in more details when we look at RIP
For the record they are:-
-Convergence
-Routing loops
-Split horizon
-Route poisoning
-Hold downs

Dynamic routing is deployed in large inter-networks
Easier to set up and manage
Routers can find their own work-a-rounds for down routes
REDUCES efficiency of WAN links
Takes a while for the routers to learn the links
Higher processing and memory requirements within the router.

Cisco Routers Ports, Interfaces and the IOS

  • Filed Under:
  • Cisco
  • 12 February 2012

Cisco Routers Ports, Interfaces and the IOS

External ports and interfaces
Routers can be purchased with different ports and interfaces.
Typical interfaces include:-
-Console port
-Auxiliary port
-Serial port

The CONSOLE port
-Starting point for configuration.
-Allows terminal connection (hyperTerminal in windows) via a PC and a cable.
-Typically basics like IP addresses will be assigned through the connection.
-Once basic configuration is in place further configuration can be done via telnet across the network.

The AUXILIARY port
-Allows a modem to be connected to the router.
-This can then be used to dial into the router for remote troubleshooting or configuration.
-Its main use is for remote configuration since troubleshooting and minor changes can be done via a telnet connection.

The SERIAL port
-Used for the connection of a leased line. IE kilo Stream or Mega Stream and carries user data.
-Represents the WAN side of the router
-Once enabled in the router’s software and given an IP address it can also be used for TELNET access into the router.
-This allows configuration and troubleshooting to be done remotely across the WAN link.

The AUI port
-Attachment unit interface
-Used for connection to the LAN
-Transceivers need to be purchased separately depending on the transmission used in your network.
-This allows the router to be transferable between different media without the need for different hardware configurations.
Internal components

Since the router is a complex device it has internal processing and file storage capabilities.
Typical components inside include:-
-ROM
-Flash
-RAM
-NVRAM

ROM – typically holds:-
-The bootstrap startup program
-Power On Self Test (POST)
-Can hold the routers Operating System on certain models.
-Generally installed on the motherboard in sockets to allow easy upgrade and replacement.

Flash
-Basically a form of Re-programmable ROM.
-It’s a non-volatile form of storage
-Not all models have flash memory
-If present will hold the IOS (routers operating system)
-Allows easy upgrade. No need to remove chips from the motherboard.

NVRAM
-Non-volatile RAM
-Stores the router’s startup configuration file
-This file is a bit like AUTOEXEC.BAT in that it contains all the instructions within the router’s configuration
-IE IP addresses for ports
-Interface descriptions
-Routes to remote networks

RAM
-Volatile memory
-Holds the running IOS
-Holds the route tables when the router is running
-Provides caching and packet buffering to increase performance

The boot procedure
When the power is turned on the router will begin its boot procedure. This progresses as follows:-
-The POST will run. This checks CPU, Memory and interface circuitry.
-The bootstrap program is initialised from ROM. This searches for the IOS.
-IOS is loaded from Flash or ROM

The bootstrap then searches for a startup configuration file
The startup configuration file is loaded from NVRAM.
If no startup configuration file is found the setup dialogue is displayed.
The internet work operating system.
The router is configured via a textual user interface known as the INTERNET WORK OPERATING SYSTEM (IOS)
This can be accessed via the console port or auxiliary port with a pc running terminal emulation software. EG HyperTerminal.
If the router already has a basic configuration we can telnet in via a network interface.

The Exec.
The IOS contains something known as the EXEC.
This is a bit like command.com in DOS.
It takes the command you enter and interprets it into an operation.
Unlike command.com the exec has different modes.
There are three exec modes:-
-User mode
-Very low risk level of access. Eg. check router status and view basic system info
-Privileged mode
-From here you can get detailed status info, use utilities like ping and access configuration mode
-Configuration mode
-Sometimes called global configuration mode.
-This mode of the exec is the highest risk.
-It allows manual configuration of the router.
-Allows access to the various interface configuration modes.

Command prompts.
You can tell which mode you are in by looking at the router prompt:-
-User mode = Router>
-Privileged mode = Router#
-Configuration mode = Router(config)#
? can be used from any mode to get help.

Configuration files
Each router has two configurations:-
-Startup configuration
-Loaded at router startup
-Stored in NVRAM and is loaded into DRAM at boot up
-Running configuration
-The configuration that is currently running
Any changes we enter in the router affect the running configuration.
This means if we want them to be permanent they need to be saved to the startup configuration.
This is useful since it allows us to make temporary changes to the router’s configuration.

Erase startup-config
-Erases the routers configuration. Basically returns it to the state it left the factory

Copy running-config startup-config
-Saves the currently running configuration to the startup-config file

The following commands are used to manage the configuration files:-
-Show startup-config
-Displays the startup configuration file
-Show running-config
-Displays the currently running configuration

Setting passwords
To set passwords you need to be in global configuration mode.
There are a number of passwords that can be used to protect your router
They are case sensitive
-Secret
-Used to restrict access to the privileged mode
-Virtual terminal
-Protects telnet access. Has to be configured for telnet access to work.
-Auxiliary
-Used to protect access from the router’s auxiliary (modem) interface.
-Console
-Used to protect direct cable access via the console port

Cisco Routers Networking Overview

  • Filed Under:
  • Cisco
  • 12 February 2012

Cisco Routers Networking Overview

Network devices
There are three key devices you need to be aware of in order to understand internet-working:-
-HUB
Used as the central connection point within the star topology.
-SWITCH
Used to reduce collision domains within a star network.
-ROUTER
Used to link network segments together

HUB
All devices on a star network plug into the hub.
Hence it forms the central connection point of the whole network.
If a hub receives a signal on a port it will propagate that signal to the whole network.
This means that collisions can result.
The more collisions on a network the slower the network will run.

SWITCHES
Switches are sophisticated hubs.
They learn what machines are connected to each port.
They only propagate the signal on the port the device is connected to.
Switches reduce collision domains.

How do switches learn?

Using Switches
Switches are too expensive for most networks to be entirely switched.
This means they get used in combination with hubs.
The idea behind this is that of a compromise. In a completely switched LAN all computers would be on their own collision domain.
When used in combination with hubs we have more pcs in the collision domain but less than if we simply had hubs.
Broadcasts are sent to the whole LAN.

A switched network

Routers
Routers are used in two instances
-If you want to reduce the broadcast domains on your network
-If you want to link LANs together via some kind of telephone network
Unlike switches, routers have to be programmed before they can function.
-The basic configuration includes things like IP addresses, routes to remote networks etc.
This will typically be done via a direct cable connection or across the network via telnet.

How do routers work?
The aim of a router is to link separate networks together.
It is only interested in the destination network. The host portion of the address is irrelevant.
If a host computer wants to send to another computer it will work out if the destination is local or remote.
-if the destination is LOCAL it will send directly to that machine
-if the destination is REMOTE it will send to its default gateway.
The default gateway is the local address of the router.
The router then calculates which network the packet is destined for.
It then examines its route tables for the route to that network.
If a route is present it will list the address of the next router (hop) in the route.
The router will then forward to that router.
If the route wasn’t present it will either be discarded or forwarded to the gateway of last resort. (This is the router version of the default gateway)

TCP/IP
Stands for Transport Control Protocol \ Internet Protocol.
Is an example of a routable protocol.
This means that it can be routed between networks.
Its configuration is made up of two parts
-an address
-a subnet mask
The address is made up of four octets.
This is because each section contains eight binary digits.
Typically we write the address in decimal.
-192.36.27.1
The address contains the network address and the address of the machine.
We therefore have to tell the system how the address is broken down.
We do this by specifying the subnet mask.
There are three main subnet masks. – one for each class of network
The sections that are 255 tell the systems that the corresponding octet in the address is part of the network address.
-192.36.27.1 mask 255.255.255.0
-Therefore network address is 192.36.27.0
-This is a class c network
A class A network would have a subnet mask at 255.0.0.0
-10.0.0.0
A class B network would have a subnet mask at 255.255.0.0
-128.37.0.0
A class C network would have a subnet mask at 255.255.255.0
-192.36.27.0

You can tell the class of network by looking at the first octet of the address.
-1-127 = Class A (127 is reserved for testing)
-128-191 = Class B
-192-223 = Class C
Although eight bits allows us up to 255 some cannot be used because of how the system works in binary.

Windows XP – System Utilities Part 3

Windows XP – System Utilities Part 3

Memory Usage.
By default more memory is given to the running of local programs.
This is typical for a workstation, If the system shares a lot of data on a network it may offer better performance if more memory is given to the system cache.
This allows more files to be cached in RAM meaning the system can share data faster.
Virtual Memory.
Virtual memory is an area of the hard disk that is set aside to store running programs and files it is sometimes referred to as the swap file or the page file, It is used when the system is running out of proper RAM.
When RAM becomes full the system will move the oldest data stored to the swap files, This is done in 4k chunks called pages.
This frees up system RAM fir new data to be stored, If the data moved to virtual memory is required later it can be pulled back from the swap file.
The size and location of the swap file can make a big difference to performance,
Generally the system will be responsible for managing the size of the page file, This can cause a slowdown as the system will have to monitor and change the size of the page file.
We can potentially speed up the system by setting the initial and maximum sizes to those recommended by windows, This means the page file will have a fixed size.
Microsoft also recommends the following
1. Never use the “No Page File Option”.
2. The page file should never be more than 3x the amount of RAM.
3. Avoid locating the page file on system drives like c:/
4. Spread the space over as many physical disks as possible.
5. Do not put page files on more than one logical drive of the same disk
6. Do not put page files on fault tolerant drives.

Windows XP – System Utilities Part 2

Windows XP – System Utilities Part 2

Task Manager.
The applications tab is useful if a program has crashed we can end and tasks that are listed as ‘not responding’
Processes show the software running at present and the resources they are using.
The only process that should be near 100% is the system idle process.
The svchost is running more than once, This is normal as svchost looks after DLL (direct library link) based software, Each instance manages a number of software services.

Performance Tweaks.
The best way to keep your system running satisfactorily is basic maintenance.
1. Defrag hard disks regularly
2. Clean out temporary files periodically
3. Ensure you have up to date anti-virus and anti-spyware utilities running
There are also some more advanced tweaks that can be made.

Advanced Performance Tweaks.
Windows runs a lot of software to make it look pretty, We can remove many of these niceties to improve performance.
Using fewer visual effects can drastically improve performance in some cases but xp will look less impressive.
Processor scheduling allows us to say which should be given CPU priority – Programs of Services
In this context the program is the active window, Services are any minimised programs.
If you multi-task frequently you may want to give priority to background services.
This gives equal CPU time to all running programs rather than giving more to the active window.

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